Examples of transparent optical materials include methacrylic resins represented by methyl methacrylate homopolymer (PMMA), polystyrene (PS), styrene/methyl methacrylate copolymer (MS), polycarbonate (PC) and the like.
In particular, methacrylic resins have been applied in such industrial fields as signboards, lighting covers, automotive parts, and ornaments because they are excellent in transparency, surface hardness, weather resistance, and the like and have good moldability. Because of small birefringence as an optical property, methacrylic resins have also been applied as optical resins for optical materials such as optical disks, optical films, and plastic substrates.
However, in recent years, with the development of various optical products, for example, flat panel displays such as liquid crystal displays, plasma displays, and organic EL displays, small infrared sensors, micro-optical waveguides, microlenses, and pick-up lenses for DVD/Blue Ray Disks handling short-wavelength light, not only excellent transparency but also high heat resistance and weather resistance, dimensional stability during moisture absorption, and control of birefringence have been required for optical resins for optical materials.
Recently, as a result of sophistication of various optical products as described above, in addition to the characteristics above, even higher homogeneity of optical properties have been required for optical materials. Specifically, in a case of a polarizing plate protective film used for a liquid crystal polarizing plate, an optical material having smaller birefringence (=retardation) even with the same total light transmittance is required. On the other hand, in a case of a quarter wave plate having a function of converting linear polarization by a polarizing plate into circularly polarized light, an optical material intended to exhibit birefringence (=retardation) of a required magnitude is required. For this purpose, it becomes necessary to control birefringence (positive/negative/zero) of an optical material or to prevent birefringence distribution in an optical material.
For example, with increase in size of flat panel displays, the display screens are more often viewed not only from the front but also sideways. In such a case, in principle, the displays have the problem of display color change or contrast reduction depending on the angle at which they are viewed. In order to improve the viewing angle characteristic, there is a demand for optical films and for a technique that controls birefringence of optical films to almost zero or to a significant positive or negative value.
As a result of the increase in size of flat display panels, the required optical materials are also increased in size. Due to biased external forces, birefringence distribution takes place in the optical materials, causing contrast reduction. Thus, an optical material having a small birefringence change caused by external forces, that is, a small absolute value of a photoelastic coefficient, has been demanded for reducing birefringence distribution (Non Patent Literatures 1 and 2).
As a related art for controlling the intrinsic birefringence of optical resins, for example, Patent Literature 1 discloses a copolymer composed of 70 to 85% by mass of a methyl methacrylate monomer unit and 15 to 30% by mass of one or two or more N-substituted maleimide monomer units, and an optical element thereof. Example of Patent Literature 1 discloses a ternary copolymer of methyl methacrylate, N-cyclohexylmaleimide, and N-o-chlorophenylmaleimide (MMA/CyMI/CIPheMI=77/20/3 wt %) and shows that intrinsic birefringence thereof is extremely small. The description contains a mention of a method of reducing optical inhomogeneity in a molded object, in which the objective is achieved by selecting a particular copolymer composition and minimizing intrinsic birefringence. On the other hand, Non Patent Literature 3 suggests that birefringence of a molded object as the optical homogeneity should include not only intrinsic birefringence of the resin used therein but also birefringence developed as a result of molding such as injection molding or extrusion molding. Specifically, according to the description that, when lenses or optical disks are obtained by injection molding, although (orientation) birefringence that reflects intrinsic birefringence can be reduced by 1) minimizing the intrinsic birefringence and by 2) selecting process conditions, birefringence distribution (=optical inhomogeneity) generally exists in a molded object because (photoelastic) birefringence remains in the vicinity of the gate due to stress strain caused by a flow of polymer chains during molding. That is, there is no disclosure that it is necessary to minimize intrinsic birefringence and to minimize a photoelastic coefficient in order to increase the optical homogeneity. There is no description as to optically perfect isotropy, either. That is, in order to increase optical homogeneity, it is additionally necessary to develop a method of minimizing a photoelastic coefficient.
Other techniques for controlling intrinsic birefringence include, for example, Patent Literatures 2 and 3. Patent Literature 2 discloses a copolymer composed of 89 to 40% by mass of a methyl methacrylate unit, 10 to 30% by mass of an aromatic vinyl compound unit, and 1 to 50% by mass of a maleimide or N-substituted maleimide unit, and a retarder plate thereof Example in Patent Literature 2 shows that a drawn sheet of a ternary copolymer of methyl methacrylate, styrene, and N-cyclohexylmaleimide (MMA/St/CyMI=80/10/10 wt %) has retardation, with small retardation unevenness, and is excellent in solvent resistance. However, the photoelastic coefficient thereof is still large and the birefringence is also large. There is no description as to optically perfect isotropy for minimizing birefringence and photoelastic coefficient at the same time.
Patent Literature 3 discloses a copolymer of 45 to 85% by mass of a (meth)acrylic acid ester unit, 10 to 40% by mass of an aromatic vinyl compound unit, and 5 to 20% by mass of an aromatic maleimide unit, and an optical drawn film thereof, in which the content of the aromatic vinyl compound unit is greater than the content of the aromatic maleimide unit. Example in Patent Literature 3 shows that a successive biaxially drawn film of a ternary copolymer of methyl methacrylate, styrene, and N-phenylmaleimide (MMA/St/PheMI=70/20/10 wt %) exhibits large negative retardation and is excellent in thermal decomposition resistance. However, photoelastic coefficient and birefringence thereof are large. There is no description as to optically perfect isotropy.
As a related art for controlling a photoelastic coefficient, for example, Patent Literature 4 discloses a method of adjusting a photoelastic coefficient to zero by combining a monomer having a photoelastic coefficient with the positive sign and a monomer with the negative sign. Specifically, Patent Literature 4 discloses a method of combining methyl methacrylate, which is a monomer having a photoelastic coefficient with the negative sign, with an unsaturated double bond-containing compound, which is a monomer having a photoelastic coefficient with the positive sign, and in which the sign of the photoelastic coefficient of a homopolymer formed is opposite to that of poly(methyl methacrylate). Patent Literature 4 describes that polystyrene, polycarbonate, a styrene/methyl methacrylate copolymer, etc., have an aromatic ring in a molecule and thus easily cause orientation strain or birefringence. Then, the listed examples of the monomer having a photoelastic coefficient with the positive sign include aliphatic hydrocarbon group methacrylates such as dodecyl methacrylate, alicyclic hydrocarbon group methacrylates such as cyclohexyl methacrylate, tricyclodecyl methacrylate, and cyclododecyl methacrylate, N-aliphatic hydrocarbon group-substituted maleimides such as N-ethylmaleimide, and N-alicyclic hydrocarbon group-substituted maleimides such as N-cyclohexylmaleimide. That is, Patent Literature 4 contains no description that suggests the characteristics of methacrylates such as benzyl methacrylate having a substituent containing an aromatic group or N-substituted maleimides such as N-phenylmaleimide, and the effects of copolymers thereof. Example in Patent Literature 4 discloses a binary copolymer of methyl methacrylate and N-cyclohexylmaleimide (MMA/CyMI=80/20 wt %) and shows that the absolute value of photoelastic coefficient thereof is less than 1.0×10−12 Pa−1. However, the copolymer has a glass transition temperature (Tg) as low as 110° C. and is insufficient in heat resistance. There is no description as to optically perfect isotropy, either.
Patent Literature 5 discloses a composition of a copolymer of 98 to 50% by mass of a methacrylate unit, 1 to 20% by mass of an arylmaleimide unit, 1 to 30% by mass of an alkylmaleimide unit, and 0 to 15% by mass of any other monomer unit, and a rubber-modified thermoplastic resin. Example in Patent Literature 5 only discloses a quarterpolymer of methyl methacrylate, styrene, N-phenylmaleimide, and N-cyclohexylmaleimide, as a copolymer. The description of Patent Literature 5 mentions that it is preferable to combine N-phenylmaleimide and N-cyclohexylmaleimide in order to improve the phase solubility with the rubber-modified thermoplastic resin, and that adjustment of the refractive index of the quarterpolymer is necessary so that the refractive index difference between a matrix portion mainly composed of the quarterpolymer and the rubber portion should be 0.01 or less in order to obtain excellent transparency. However, there is no mention as to the optical properties (intrinsic birefringence, photoelastic coefficient), and there is no suggestion as to birefringence control and photoelastic coefficient control nor description as to optically perfect isotropy. There is also a problem of a large photoelastic coefficient of the copolymer.
As a related art for optically perfect isotropy for minimizing birefringence and photoelastic coefficient at the same time, for example, Non Patent Literatures 3 and 4 disclose a methyl methacrylate/methacrylic acid-2,2,2-trifluoroethyl/benzyl methacrylate ternary copolymer (=52/42/6 wt %). There has been a problem in that this acrylic thermoplastic resin is insufficient in heat resistance although birefringence and photoelastic coefficient thereof can be controlled at the same time and the absolute values of birefringence and photoelastic coefficient thereof can be controlled to zero at the same time (zero-zero birefringence).
Patent Literature 6 discloses a quarterpolymer of methyl methacrylate, styrene, maleic anhydride, and benzyl methacrylate (MMA/St/MAH/BzMA=60/12/27/1 wt %) as an acrylic thermoplastic resin exhibiting optically perfect isotropy and high heat resistance. However, there has been a problem in that the long-time stability of optical properties is low in hot and humid environments (80° C., 90% RH).
As other related arts, for example, Patent Literatures 7 to 9 disclose a ternary copolymer of methyl methacrylate, N-cyclohexylmaleimide, and benzyl methacrylate. There has been a problem in that this acrylic thermoplastic resin is not always sufficient in heat resistance although low birefringence can be controlled to some degree.
In summary, in the range of related arts, there exists no technique that can provide an acrylic thermoplastic resin excellent in heat resistance, weather resistance, and low water absorbency, in which birefringence thereof (birefringence and photoelastic coefficient) serving as an optical property is highly controlled.